Historically, the gears of automatic transmissions for passenger vehicles have been changed by using a shifter in the passenger cabin that is connected to the transmission by a mechanical linkage. The driver typically grasps and moves the shifter to change gears. When shifting between particular gears, e.g., from Park to Reverse, the user must also simultaneously depress a button in order to move the shifter. Recently, electronically controlled shifting systems, commonly referred to as shift-by-wire systems, have been developed. Shift-by-wire systems have an electronic connection to the transmission that effects the changing of gears. In a typical shift-by-wire system, the driver presses a button to select a desired gear, sometimes depressing an additional button in order to shift between particular gears, e.g., from Park to Reverse. An electronic signal is then sent to an actuator that effects shifting of the transmission. Shift-by-wire systems can reduce the space required for the shifting system, reduce assembly problems, decrease costs and increase effectiveness. U.S. Pat. No. 6,196,078 and U.S. Pat. No. 6,295,887 to DeJonge et al., U.S. Pat. No. 4,892,014 to Morell et al., and U.S. Pat. No. 5,035,158 to Leigh-Monstevens all describe shift-by-wire actuators for automatic transmissions.
Many of the known shift-by-wire systems, however, are limiting in that they do not provide a manual override capability for operation of the transmission in the event of loss of power. In certain shift-by-wire systems, a battery backup unit must be provided in order to ensure that the transmission can be shifted in the event of the loss of power, such as when the vehicle needs to be towed.
It is an object of the present invention to provide a shift-by-wire actuator that reduces or wholly overcomes some or all of the difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable or experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.